Aftertreatment system

ABSTRACT

An aftertreatment system for an internal combustion engine includes a treatment device having a combined particulate filter and SCR catalyst. The combined particulate filter and SCR catalyst treats uncatalyzed exhaust from the internal combustion engine including between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr.

TECHNICAL FIELD

The present disclosure is directed to an aftertreatment system and, more particularly, to an aftertreatment system including a selective catalytic reduction catalyst.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of particulates and gaseous compounds including, among other things, oxides of nitrogen (NO_(X)). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amounts of particulates and NO_(X) emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.

In order to comply with the regulation of particulates and NO_(X), some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is a process where a reductant, most commonly urea ((NH₂)₂CO) or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and absorbed onto a downstream substrate. The injected urea solution decomposes into ammonia (NH₃), which reacts with NO_(X) in the exhaust gas to form water (H₂O) and diatomic nitrogen (N₂). Engine manufacturers implementing the SCR process typically include an oxidation catalyst upstream of the SCR substrate to assist in altering the composition of the exhaust gas stream before it passes to the SCR substrate. Such oxidation catalysts typically include a porous substrate made from, coated with, or otherwise including a catalyzing material such as palladium, platinum, vanadium, and/or other precious metals. Such materials facilitate a conversion of NO to NO₂, thereby increasing the ratio of NO₂ to NO upstream of the SCR substrate. The elevated level of NO₂ provided by the oxidation catalyst may assist in both improving NOx conversion over the SCR catalyst and oxidizing soot particles that collect in a particulate filter.

In some applications, the substrate used for SCR purposes may need to be very large to help ensure it has enough surface area or effective volume to absorb appropriate amounts of the ammonia required for sufficient catalytic reduction of NO_(X). These large substrates can be expensive and require significant amounts of space within the exhaust system. In addition, the substrate must be placed far enough downstream of the injection location for the urea solution to have time to decompose into ammonia and to evenly distribute within the exhaust flow for the efficient reduction of NO_(X). Due to the size of the SCR substrate and the required spacing between the substrate and the injector, packaging an exhaust system utilizing such components can be difficult, and packaging an exhaust system utilizing an oxidation catalyst upstream of the SCR substrate can be even more difficult. In addition, due to the precious metals used to manufacture oxidation catalysts, utilizing an oxidation catalyst can significantly increase the cost of the exhaust system.

An exemplary SCR-equipped system for use with a combustion engine is disclosed in JP Patent Publication No. 2008/274,851 (the '851 publication) of Makoto published on Nov. 13, 2008. This system includes an exhaust gas purification device having a gas accumulation canister, a separate dispersion canister, and a mixing pipe connected between the gas accumulation and gas dispersion canisters. A particulate filter and an oxidation catalyst are disposed in the gas accumulation canister, while an SCR catalyst and ammonia reduction catalyst are disposed within the gas dispersion canister. A urea injector is located in the mixing pipe, upstream of the SCR catalyst.

Although the exhaust system of the '851 patent may be configured to treat exhaust gases, such a system may be problematic in many aftertreatment applications. In particular, the multiple canisters and catalysts used in the '851 system may increase the cost, packaging complexity, and an overall size of the system.

The aftertreatment systems of the present disclosure solve one or more of the problems set forth above and/or other problems of the prior art.

SUMMARY

In an exemplary embodiment of the present disclosure, an aftertreatment system for an internal combustion engine includes a treatment device having a combined particulate filter and SCR catalyst. The combined particulate filter and SCR catalyst treats uncatalyzed exhaust from the internal combustion engine including between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr.

In another exemplary embodiment of the present disclosure, an aftertreatment system for an internal combustion engine includes a treatment device having a first SCR catalyst disposed on a particulate filter substrate, and a second SCR catalyst downstream of the first SCR catalyst. The first SCR catalyst treats uncatalyzed exhaust from the internal combustion engine. The treatment device is characterized by a NOx conversion efficiency greater than approximately 95 percent.

In an additional exemplary embodiment of the present disclosure, an exhaust treatment method includes generating exhaust with an internal combustion engine, the exhaust including between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr. The method also includes directing the exhaust uncatalyzed from the internal combustion engine to a combined particulate filter and SCR catalyst. The method further includes catalytically reducing NOx in the exhaust with the combined particulate filter and SCR catalyst, the combined particulate filter and SCR catalyst forming a first treated exhaust.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial schematic view of an aftertreatment system according to an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

An exemplary aftertreatment system 10 is shown in FIG. 1. The aftertreatment system 10 may include one or more treatment devices 28 connected to an internal combustion engine 22, such as, for example, a diesel engine. The engine 22 may include an exhaust line 23 directing exhaust generated by the engine 22 to the treatment device 28. The engine 22 may also include a turbocharger 18 connected to the exhaust line 23, and a compressor 20 driven by the turbocharger 18 via one or more rotating shafts 19. In such an embodiment, the treatment device 28 may be fluidly connected to an outlet of the turbocharger 18.

The treatment device 28 may include one or more canisters 12 fabricated from a one or more corrosion-resistant materials. Such materials may include, for example, stainless steel or other like metals. In further exemplary embodiments, such materials may be treated, coated, and/or other wise provided with corrosion protection. In the embodiment shown in FIG. 1, the canister 12 includes a single inlet 14 and a single outlet 16. It is contemplated, however, that the treatment device 28 may include any number of inlets and outlets, as desired.

As shown in FIG. 1, the treatment device 28 may include one or more filters, catalysts, and/or combinations thereof to assist in treating components of the flow of exhaust gas 44. For example, the treatment device 28 may include a combined diesel particulate filter and SCR catalyst 30 (hereinafter referred to as “CDS catalyst 30”) disposed within the canister 12. As will be described in greater detail below, the CDS catalyst 30 may include a single filtration media that is configured to form both particulate trapping and SCR functions. In additional exemplary embodiments, the CDS catalyst 30 may alternatively be replaced with a separate and dedicated particulate filter and SCR catalyst, if desired. The separate filter and SCR catalyst may be disposed within the canister 12 or, alternatively, may be disposed within separate canisters.

In further embodiments, the treatment device 28 may include an additional catalyst 32 disposed within the canister 12 downstream of the CDS catalyst 30. The additional catalyst 32 may be, for example, an SCR catalyst. In additional exemplary embodiments, the additional catalyst 32 may include an upstream region 32A that functions as an SCR catalyst, and a downstream region 32B that functions as a cleanup catalyst such as, for example, a diesel oxidation catalyst or an ammonia oxidation catalyst. In an alternative embodiment, the additional catalyst 32 may be a dedicated cleanup catalyst (e.g., catalyst 32 may not provide SCR functionality).

The CDS catalyst 30 may be configured to perform particulate trapping functions. In particular, CDS catalyst 30 may include filtration media configured to remove particulate matter from an exhaust flow. In one embodiment, the filtration media of the CDS catalyst 30 may embody a generally cylindrical deep-bed type of filtration media configured to accumulate particulate matter throughout a thickness thereof in a substantially homogenous manner. The filtration media may include a low density material having a flow entrance side and a flow exit side, and may be formed through a sintering process from metallic or ceramic particles. It is contemplated that the filtration media may alternatively embody a surface type of filtration media fabricated from metallic or ceramic foam, a wire mesh, or any other suitable material.

The CDS catalyst 30 may also be configured to perform SCR functions. Specifically, the filtration media of CDS catalyst 30 may be fabricated from or otherwise coated with a ceramic material such as titanium oxide, a base metal oxide such as vanadium and tungsten, zeolites, and/or a precious metal. With this composition, decomposed reductant entrained within an exhaust flow passing through the CDS catalyst 30 may be absorbed onto the surface of and/or within the filtration media, where the reductant may react with NOx (NO and NO₂) in the exhaust gas to form water (H₂O) and diatomic nitrogen (N₂). It is contemplated that CDS catalyst 30 may perform both particulate trapping and SCR functions throughout the media of CDS catalyst 30 or, alternatively, in serial stages, as desired. If NO₂ levels within the exhaust are sufficiently high, the exothermic reaction between the reductant and the NOx may assist in passively regenerating the CDS catalyst 30.

As described above, the additional catalyst 32 may comprise an upstream region 32A and a downstream region 32B. In particular, a single substrate brick of catalyst 32 may include an upstream region (32A) located proximate and/or adjacent the CDS catalyst 30. The upstream region 32A may be fabricated from or otherwise coated with a material that absorbs reductant onto its surface or otherwise internalizes reductant for reaction with NOx (NO and NO₂) in the exhaust gas passing therethrough. Such a reaction may form water (H₂O) and diatomic nitrogen (N₂). Similarly, the substrate brick of catalyst 32 may include a downstream region (32B) located proximate and/or adjacent the outlet 16. The downstream region 32B may be disposed adjacent to and downstream of the upstream region 32A, and may be coated with or otherwise contain a different catalyst than the upstream region 32A. Such catalysts may include an oxidation catalyst configured to oxidize residual reductant in the exhaust.

In exemplary embodiments in which the downstream region 32B of the additional catalyst 32 comprises an oxidation catalyst, such an exemplary oxidation catalyst may be, for example, a diesel oxidation catalyst (DOC) or an ammonia oxidation (AMOx) catalyst. Such oxidation catalysts may comprise any suitable substrate coated with or otherwise containing a catalyzing material, for example a precious metal, that catalyzes a chemical reaction to alter a composition of exhaust passing through the oxidation catalyst. In one embodiment, such an oxidation catalyst may include palladium, platinum, vanadium, or a mixture thereof that facilitates oxidation of residual ammonia gas and/or entrained reductant. Such catalysts may also facilitate the oxidation of NO in the exhaust to NO₂. In another embodiment, the additional catalyst 32 may alternatively or additionally perform particulate trapping functions (e.g., the additional catalyst 32 may include a particulate trap such as a continuously regenerating technology particulate filter or a catalyzed continuously regenerating technology particulate filter), hydrocarbon oxidation functions, carbon monoxide oxidation functions, and/or other functions known in the art.

As shown in FIG. 1, a gap 34 may be maintained at the opposing ends of canister 12, proximate the inlet 14 and outlet 16, respectively. A gap 34 may also be maintained between the CDS catalyst 30 and the additional catalyst 32. The gaps 34 may act as manifolds that facilitate substantially equal distribution of exhaust across faces of the respective catalysts 30, 32 disposed within the canister 12.

It is contemplated that access to the catalysts 30, 32 of the aftertreatment system 10 may be helpful in some situations. Thus, in exemplary embodiments, end portions 46, 48 of the canister 12 enclosing the gaps 34 at the inlet 14 and outlet 16, respectively, may be removably connected to a center portion of canister 12 that encloses the CDS catalyst 30 and the additional catalyst 32. For example, the end portions 46, 48 may be bolted or latched to the center portion, if desired. With this configuration, the end portions 46, 48 may be selectively removed for inspection and/or replacement of the various catalysts 30, 32.

One or more removable couplings 26 may be connected to the end portions 46, 48 to facilitate the selective removal of the treatment device 28 from the aftertreatment system 10. Such couplings 26 may comprise any removable air-tight coupling device known in the art. In an exemplary embodiment, such couplings 26 may comprise one or more clamps, brackets, pieces of flexible tubing, and/or other like devices configured to facilitate a removable connection between the canister 12 and other components of the aftertreatment system 10. In further exemplary embodiments, the couplings 26 may embody cobra-head type couplings that are capable of bending through an angle of about 90 degrees. Such couplings 26 may have an elliptical opening at the canister 12 and a circular opening at an opposite end of the coupling 26. In still further embodiments, other types of couplings may be utilized, if desired.

As shown in FIG. 1, the aftertreatment system 10 may include a mixing tube 24 connected to the inlet 14 via one or more of the couplings 26 described above. The aftertreatment system 10 may also include a reductant injector 36 fluidly connected to the mixing tube 24. In exemplary embodiments, the reductant injector 36 may be disposed upstream of the treatment device 28 (e.g., within an upstream end of the mixing tube 24 or within one of the coupling 26) and configured to inject a reductant into the exhaust flowing through the mixing tube 24. A gaseous or liquid reductant, most commonly a water/urea solution, ammonia gas, liquefied anhydrous ammonia, ammonium carbonate, an amine salt, or a hydrocarbon such as diesel fuel, may be sprayed or otherwise advanced by the reductant injector 36 into the exhaust passing through the mixing tube 24. For example, the reductant injector 36 may be disposed at any desired distance upstream of the CDS catalyst 30 to allow the injected reductant sufficient time to mix with exhaust and to sufficiently decompose before entering the CDS catalyst 30. In exemplary embodiments, an even distribution of sufficiently decomposed reductant within the exhaust passing through the CDS catalyst 30 may enhance NO_(X) reduction therein. The distance between the reductant injector 36 and the inlet 14 of the canister 12 (e.g., the length of the mixing tube 24) may be based on a flow rate of exhaust passing through aftertreatment system 10 and/or on a cross-sectional area of the mixing tube 24. In some exemplary embodiments, the distance between the reductant injector 36 and the inlet 14 of the canister 12 may be 2 feet or more.

To enhance incorporation of the reductant with the exhaust, a mixer 38 may be disposed within the mixing tube 24. In an exemplary embodiment, the mixer 38 may include vanes or blades inclined to generate a swirling motion of the exhaust as it flows through the mixing tube 24. In another exemplary embodiment, the mixer 38 may include a ring extending from internal walls of the mixing tube 24 radially inward a distance toward a longitudinal axis of the mixing tube 24. Such a ring may be configured to promote exhaust flow turbulence within the mixing tube 24, thereby assisting in incorporating the reductant into the exhaust. In either embodiment, the mixer 38 may be disposed upstream or downstream (shown in FIG. 1) of the reductant injector 36.

The aftertreatment system 10 may also include one or more probes situated to monitor operating characteristics and/or other parameters of the aftertreatment system 10. For example, a first probe 40 may be situated within the gap 34 proximate the inlet 14 upstream of the CDS catalyst 30. In addition, a second probe 42 may be situated within the gap 34 proximate the outlet 16 downstream of the additional catalyst 32. In one embodiment, first probe 40 may be a temperature probe configured to generate a first signal indicative of a temperature of the exhaust entering CDS catalyst 30. The first signal may be utilized by a controller (not shown) to determine, among other things, an operating temperature and predicted efficiency of the CDS catalyst 30. The second probe 42 may be utilized to detect a constituent of the exhaust exiting catalyst 32, for example a concentration of NOx or an amount of residual reductant. The second probe 42 may generate a second signal indicative of this constituent, and the second signal may be utilized to determine, among other things, an actual effectiveness of the CDS catalyst 30 and/or the additional catalyst 32. It is contemplated that at least one of the probes 40, 42 may be configured to monitor other parameters of the aftertreatment system 10, and may be utilized for other purposes, if desired.

INDUSTRIAL APPLICABILITY

The aftertreatment system 10 of the present disclosure may be applicable to any engine configuration requiring the treatment of exhaust where component packaging is an important issue. While known aftertreatment systems utilize a DOC catalyst upstream of an SCR catalyst for converting NO to NO₂, such DOC catalysts are typically disposed in large canisters, and due to the precious metals utilized in DOC catalysts, such catalysts are costly. The aftertreatment system 10 of the present disclosure, on the other hand, operates without using a DOC catalyst upstream of the treatment device 28. As a result, the disclosed system 10 takes up less space downstream of the engine 22, and is less expensive, less complicated, and easier to package on vehicles utilizing the engine 22 than known systems.

To compensate for the conversion of NO to NO₂ provided by the upstream DOC catalyst of known aftertreatment systems, the engine 22 of the present disclosure may be calibrated to generate exhaust having increased NOx levels (thereby increasing the amount of NO₂ in the exhaust) and decreased soot levels. For example, the timing of in-cylinder fuel injections may be advanced, the pressure of such injections may be increased, the flow of recirculated exhaust gas into the engine 22 may be reduced or eliminated, and/or the throughput of the compressor 20 and/or the turbocharger 18 may be increased in order to facilitate generating such exhaust. The elevated NOx levels resulting from such engine calibration may ensure passive regeneration of the CDS catalyst 30 due to the exothermic reduction reaction at the SCR catalyst contained therein, and may also increase the fuel efficiency of the engine 22. Such NOx levels may be, for example, between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr. In further exemplary embodiments, such NOx levels may be greater than approximately 10 g NOx/kW-hr. Due to such calibration, the engine 22 may generate exhaust having a NO₂ to NO ratio of approximately 1 to 2. In addition, such engine calibration may result in an improvement in fuel efficiency and a reduction in soot production by the engine 22. The exhaust flow through the aftertreatment system 10 will now be described.

Referring to FIG. 1, the engine 22 may generate uncatalyzed exhaust 44 containing a complex mixture of air pollutants including, among other things, NO_(X) and soot. The exhaust 44 may pass from the engine 22, through the turbocharger 18, via the exhaust line 23. The uncatalyzed exhaust 44 may then enter the mixing tube 24, where reductant may be injected into the exhaust 44 by the reductant injector 36 upstream of mixer 38. The swirl and/or turbulence of the exhaust 44 promoted by mixer 38 may be utilized to entrain and distribute reductant within the exhaust 44. As the swirling and/or turbulent flow of exhaust and reductant passes along the length of the mixing tube 24, the reductant may continue to homogenize within the uncatalyzed exhaust 44, and the reductant may begin to decompose into ammonia. Thus, the length and location of the mixing tube 24, together with the mixer 38, may promote decomposition of the injected reductant.

The uncatalyzed exhaust 44 may be directed from the mixing tube 24 into the canister 12 via the inlet 14. The exhaust 44 may flow from inlet 14 into the gap 34 upstream of CDS catalyst 30, and due to the change in cross-sectional area and/or volume between the mixing tube 24 and the end portion 46, the uncatalyzed exhaust 44 may expand upstream of the CDS catalyst 30. Such expansion may facilitate a substantially equal distribution of the exhaust 44 across a face of the CDS catalyst 30. By the time the exhaust 44 reaches the CDS catalyst 30, the bulk of the reductant may be decomposed, thereby facilitating NOx reduction within the CDS catalyst 30 and the additional catalyst 32.

The CDS catalyst 30 may treat the uncatalyzed exhaust 44 as the exhaust 44 passes through the CDS catalyst 30. For example, particulate matter may be removed from the exhaust 44, and NOx within the exhaust 44 may react with the reductant at the SCR catalyst. In particular, the exhaust 44 may be catalytically reduced by the SCR catalyst to form water and diatomic nitrogen. As a result, the CDS catalyst 30 may form a first treated exhaust from the uncatalyzed exhaust 44. Such a first treated exhaust may be, for example, exhaust that has undergone a catalytic reduction process in which NO has been formed from NO₂. The first treated exhaust may exit the CDS catalyst 30 and enter the additional catalyst 32, where catalytic reduction of NOx contained in the first treated exhaust may occur and residual reductant carried by the first treated exhaust may be absorbed. For example, in embodiments in which the additional catalyst 32 comprises an SCR catalyst, the SCR catalyst may catalytically reduce the first treated exhaust. As a result, the SCR catalyst of the additional catalyst 32 may form a second treated exhaust from the first treated exhaust. Such a second treated exhaust may be, for example, treated exhaust that has undergone an additional catalytic reduction process in which NO has been formed from NO₂.

In exemplary embodiments, the treatment device 28 may be characterized by a NOx conversion efficiency greater than approximately 95 percent. As used herein with regard to the SCR catalysts of the treatment device 28, the term “NOx conversion efficiency” means the percentage of NOx contained by the exhaust that is catalytically reduced to N₂ upon passing through the SCR catalysts or the treatment device 28. Further, unless otherwise specified, the NOx conversion efficiency values discussed herein are associated with treatment devices at or near the beginning of their useful life. In exemplary embodiments, the CDS catalyst 30 may be characterized by a NOx conversion efficiency of approximately 90 percent or less. The CDS catalyst 30 may provide such a NOx conversion efficiency while producing advantageous levels of backpressure upstream of the treatment device 28 and while having a size suitable for packaging in the aftertreatment system 10 of the engine 22. In such exemplary embodiments, the SCR catalyst of the additional catalyst 32 may be characterized by a NOx conversion efficiency of at least approximately 50 percent. In further exemplary embodiments, the SCR catalyst of the additional catalyst 32 may be characterized by a NOx conversion efficiency of between approximately 50 percent and approximately 80 percent. Thus, the respective NOx conversion efficiencies of the SCR catalysts may combine to result in a NOx conversion efficiency of the treatment device 28 greater than approximately 95 percent. Such NOx conversion efficiency levels may be required to comply with exhaust emission standards.

In addition, exemplary treatment device embodiments including a CDS catalyst 30 having a NOx conversion efficiency of approximately 90 percent or less, and an additional catalyst 32 having an SCR catalyst with a NOx conversion efficiency of between approximately 50 percent and approximately 80 percent, may be capable of oxidizing a greater amount of soot per unit volume of exhaust than, for example, a treatment device including a single CDS catalyst having a NOx conversion efficiency of approximately 95 percent. It is understood that, for example, passive soot regeneration may improve inversely with NOx conversion to N₂. This relationship is a result of NO₂ being consumed by the passive soot oxidation reaction and the NOx reduction reactions taking place at the one or more SCR catalysts of the treatment device 28. For example, lower levels of NOx reduction at the CDS catalyst 30 may allow for higher levels of soot reduction on the substrate of the CDS catalyst 30. Such NOx reduction may be furthered and/or completed at the additional catalyst 32 in order to achieve a NOx conversion efficiency of the treatment device greater than approximately 95 percent.

In embodiments in which the additional catalyst 32 further comprises one of a DOC catalyst and an AMOx catalyst, the second treated exhaust may pass from the SCR catalyst of the additional catalyst 32 to the downstream oxidation catalyst of the additional catalyst 32. The oxidation catalyst may catalytically oxidize the second treated exhaust. In particular, as the second treated exhaust passes through the oxidation catalyst, residual reductant entrained within the second treated exhaust may be oxidized. After treatment within the additional catalyst 32, the exhaust may pass through the gap 34 proximate the end portion 48, and may be discharged to the atmosphere or other downstream exhaust system components via the outlet 16.

The aftertreatment system 10 may be simple, compact, and relatively inexpensive. For example, the aftertreatment system 10 may be simple and compact because it may utilize only a single canister having catalysts that provide multiple functions. In addition, the CDS catalyst 30 may provide both particulate trapping and NOx reduction functionality, while the additional catalyst 32 may provide NOx reduction and oxidation functionality. The simplicity of the aftertreatment system 10 may result in a lower cost solution to exhaust aftertreatment and may require less packaging space than known systems.

It will be apparent to those skilled in the art that various modifications and variations can be made to the aftertreatment system 10 of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the aftertreatment system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalent. 

1. An aftertreatment system for an internal combustion engine, comprising: a treatment device including a combined particulate filter and SCR catalyst, the combined particulate filter and SCR catalyst treating uncatalyzed exhaust from the internal combustion engine comprising between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr.
 2. The aftertreatment system of claim 1, further comprising a reductant injector disposed upstream of the treatment device and configured to inject a reductant into the uncatalyzed exhaust.
 3. The aftertreatment system of claim 2, further comprising a mixing tube connected to an inlet of the treatment device, the reductant injector being fluidly connected to the mixing tube.
 4. The aftertreatment system of claim 3, further comprising a mixer disposed within the mixing tube.
 5. The aftertreatment system of claim 1, further comprising an additional catalyst disposed downstream of the combined particulate filter and SCR catalyst.
 6. The aftertreatment system of claim 5, wherein the additional catalyst comprises an SCR catalyst.
 7. The aftertreatment system of claim 6, wherein the additional catalyst further comprises one of an ammonia oxidation catalyst and a diesel oxidation catalyst.
 8. The aftertreatment system of claim 5, wherein the combined particulate filter and SCR catalyst and the additional catalyst are disposed within a single canister.
 9. An aftertreatment system for an internal combustion engine, comprising: a treatment device including a first SCR catalyst disposed on a particulate filter substrate, and a second SCR catalyst downstream of the first SCR catalyst, the first SCR catalyst treating uncatalyzed exhaust from the internal combustion engine, wherein the treatment device is characterized by a NOx conversion efficiency greater than approximately 95 percent.
 10. The aftertreatment system of claim 9, wherein the first and second SCR catalysts are disposed within a single canister, the canister including at least one removable end portion configured to provide access to at least one of the first and second SCR catalysts.
 11. The aftertreatment system of claim 9, wherein and the first SCR catalyst is characterized by a NOx conversion efficiency of approximately 90 percent or less.
 12. The aftertreatment system of claim 9, wherein the second SCR catalyst is characterized by a NOx conversion efficiency between approximately 50 percent and approximately 80 percent.
 13. The aftertreatment system of claim 9, further comprising one of an ammonia oxidation catalyst and a diesel oxidation catalyst downstream of the second SCR catalyst.
 14. The aftertreatment system of claim 9, further comprising a substrate having an upstream region and a downstream region, the second SCR catalyst being disposed on the upstream region of the substrate, and one of an ammonia oxidation catalyst and a diesel oxidation catalyst being disposed on the downstream region of the substrate.
 15. The aftertreatment system of claim 9, further comprising a mixing tube connected to an inlet of the treatment device, and a reductant injector fluidly connected to the mixing tube upstream of the inlet.
 16. The aftertreatment system of claim 15, further comprising a mixer disposed within the mixing tube downstream of the reductant injector and upstream of the inlet.
 17. An exhaust treatment method, comprising: generating exhaust with an internal combustion engine, the exhaust comprising between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr; directing the exhaust uncatalyzed from the internal combustion engine to a combined particulate filter and SCR catalyst; and catalytically reducing NOx in the exhaust with the combined particulate filter and SCR catalyst, the combined particulate filter and SCR catalyst forming a first treated exhaust.
 18. The method of claim 17, further comprising catalytically reducing the first treated exhaust with an additional SCR catalyst disposed downstream of the combined particulate filter and SCR catalyst, the additional SCR catalyst forming a second treated exhaust.
 19. The method of claim 18, further comprising catalytically oxidizing the second treated exhaust with one of an ammonia oxidation catalyst and a diesel oxidation catalyst.
 20. The method of claim 17, wherein the uncatalyzed exhaust generated by the engine is characterized by a NO₂ to NO ratio of approximately 1 to
 2. 